Update the README with Coq version support

Author: tchajed

README.md

@@ -52,8 +52,8 @@ There are a few files that are optional reading which make the tutorial work:
 
 ## Compiling
 
-This development relies on a development version of Iris and Coq 8.17 or later.
-We test Coq 8.17, 8.18, and master with Iris dev in CI. (The released
+This development relies on a development version of Iris and Coq 8.18 or later.
+We test 8.18, 8.19, and master with Iris dev in CI. (The released
 version of Iris is currently incompatible.)
 
 You'll need to install Iris, which is easiest done through opam. There are